We report a newborn with hyperekplexia and uncontrolled tonic spasms which did not respond to intravenous phenobarbitone and phenytoin, and midazolam infusion. Serum biochemistry, electrocardiography, electroencephalography, lumbar puncture and neuroimaging were normal. Continuous cardiac monitoring revealed that tonic spasm episodes were accompanied by sinus node paucity and severe bradycardia. Duration and number of tonic spasm episodes decreased with clonazepam therapy, and she was discharged. At 4 months of age sudden infant death occurred. Sudden infant death could be related to the paucity of sinus node. Cardiac pacemaker implantation should be considered even if the medical treatment is successful.